AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The distance from the lens to the principal focus is called the focal…
Full specification wording
Lenses (physics only): The distance from the lens to the principal focus is called the focal length.
The focal length is measured from the centre of the lens to the principal focus, along the principal axis, and it is the single number that says how strongly a lens bends light.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 3 of 9
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The focal length is measured from the centre of the lens to the principal focus, along the principal axis, and it is the single number that says how strongly a lens bends light. A short focal length means a powerful, strongly curved lens that brings rays to a focus quickly; a long focal length means a flatter, weaker lens. A convex lens has a real principal focus on each side, both at the same focal length. You can find the focal length of a convex lens using a distant object such as a window: the rays arriving are effectively parallel, so the sharp image formed on a screen lies at the principal focus and the lens to screen distance is the focal length. The focal length also decides where the image of a nearer object forms.
What earns the marks
- Measuring the focal length from the centre of the lens to the principal focus.
- Using a distant object so that the incoming rays are effectively parallel.
- Taking the lens to screen distance for the sharpest image as the focal length.
- Linking a shorter focal length to a more strongly curved, more powerful lens.
Common mistakes and how to put them right
Measuring from the edge or the surface of the lens rather than from its centre.
Confusing the focal length with the distance from the lens to the object.
Measuring to a blurred image because the screen was not adjusted to the sharpest point.
Saying the focal length changes when the object is moved.
Exam tips
- State where you measure from and to, because both ends of the measurement can carry marks.
- In a practical question, say that you would repeat the measurement and take a mean.
- Remember the focal length is a property of the lens and does not depend on the object.
Try the lesson
The start of the Lenses (physics only): The distance from the lens to the principal focus is called the focal length. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the name given to the distance from a lens to its principal focus?
Listen
The focal length is measured from the centre of the lens to the principal focus, along the principal axis, and it is the single number that says how strongly a lens bends light. A short focal length means a powerful, strongly curved lens that brings rays to a focus quickly; a long focal length means a flatter, weaker lens. A convex lens has a real principal focus on each side, both at the same focal length. You can find the focal length of a convex lens using a distant object such as a window: the rays arriving are effectively parallel, so the sharp image formed on a screen lies at the principal focus and the lens to screen distance is the focal length. The focal length also decides where the image of a nearer object forms. A strong answer earns marks for these points. measuring the focal length from the centre of the lens to the principal focus. using a distant object so that the incoming rays are effectively parallel. taking the lens to screen distance for the sharpest image as the focal length. linking a shorter focal length to a more strongly curved, more powerful lens. Exam tip: State where you measure from and to, because both ends of the measurement can carry marks.
See it
Put it in order
Put these steps in the order they happen.
- 1. This distance is the focal length
- 2. Measure from centre of lens to principal focus
- 3. Convex lens
- 4. Image lies at the principal focus
- 5. Distant object
- 6. Incoming rays effectively parallel
- 7. Screen adjusted to sharpest image
Key words
Say what each word means before you turn the card.
focal length
Try answering aloud or in your head before turning the card.
centre of the lens
Try answering aloud or in your head before turning the card.
See every definition
- focal length
- The distance from the lens to the principal focus, measured from the centre of the lens to the principal focus.
- centre of the lens
- The point from which the focal length is measured, rather than from the edge or the surface of the lens.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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